The invention relates to a valve mechanism, in particular for internal combustion engines of motor vehicles, with a driven cam element and with a valve actuator displaceable or pivotable by the cam element.
Valve mechanisms for the valve control of internal combustion engines, in particular for motor vehicles, conventionally have a device (spring, hydraulic element, etc.), by means of which the valve is acted upon in the closing position. At least during the open phase, therefore, the valve actuator (valve tappet, drag lever, rocker arm or the like) is pressed against part of a closed valve control surface, the part being eccentric to the shaft axis. During the closing of the valve, care must be taken to ensure that the valve disk does not strike the valve seat too quickly, since it otherwise rebounds. This requires relatively complicated coordination between the masses to be moved, the forces arising, the material properties, etc.
There has therefore been no lack of proposals for guiding the valve actuator positively on the cam element, various embodiments having been developed, which are each based on two eccentric valve control surfaces instead of the return spring. Actual versions may be gathered, for example, from GB patent specification 19,193 (1913) and GB patent specification 434,247, wherein the cam element has, on at least one end face, a groove, the two side walls of which form the valve control surfaces. A roller or the like arranged at the end of the valve actuator engages into the groove from the side. A cam element having a surrounding web is known, for example, from European patent publication EP 429,277.
Further examples of positive guides make use, instead of valve control surfaces parallel to the axis of rotation of the camshaft, of two valve control surfaces which are arranged axially one behind the other and are formed on two differently shaped camshaft elements, for example European patent publications EP 355 659 B, and EP 384 361 A, etc.
Both the first version of the positive guides with end-face grooves and the lateral engagement of the sensing element and the second version with two cam elements and two sensing elements for each valve have, as compared with the first-mentioned valve mechanism with a return spring, an increased axial extent and a greater number of components which present either structural, spatial or economic problems.
It is accordingly an object of the invention to provide a desmodromic valve mechanism of the type mentioned above, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which appreciably reduces the prior art problems and constitutes a space-saving, lightweight and cost-effective design.
With the foregoing and other objects in view there is provided, in accordance with the invention, a valve mechanism, comprising:
at least one driven cam element;
a flexible surround element formed with a plurality of plates and joint pins connecting the plates, the cam element being rotatably disposed in the flexible surround element about an axis of rotation; and
a valve actuator driven by the cam element, the valve actuator being movably connected to the flexible surround element for movement of the valve actuator in a plane perpendicular to the axis of rotation of the cam element.
In accordance with an added feature of the invention, the flexible surround element is a roller chain formed with the plates and the joint pins.
In other words, the objects of the invention are achieved, according to the invention, in that the cam element is rotatably disposed within a flexible surround element (e.g. a chain) which is connected movably to the valve actuator in a plane perpendicular to the axis of rotation of the cam element.
The surround element surrounds the circumference of the cam element without appreciable play, so that it is matched to the cam shape, and, because of the nature of the surround element, the cam element can rotate in the latter. Since the surround element connected to the valve actuator cannot co-rotate with the cam element, the travel of the cam region about the axis of rotation of the cam element is converted into a lifting or to-and-fro movement of the valve actuator mounted displaceably or pivotably in the cylinder head. The valve actuator does not execute any movement as long as the region of connection of the surround element to the valve actuator comes to bear against the base-circle region of the rotating cam element, is then moved radially away from the axis of rotation of the cam element and is finally led back again, while the cam region of the cam element passes the region of connection of the surround element to the valve actuator.
Since the surround element comes to bear, essentially free of play, against the circumferential surface of the cam element, a valve actuator freely projecting from the surround element would always be oriented perpendicularly to the tangent to the circumferential surface of the cam element and at the same time deviate from radial orientation to the axis of rotation of the cam element, on the one hand, in the rising cam region and, on the other hand, in the falling cam region. The movable connection of the surround element to the valve actuator permits the pivoting or tilting movement of the surround element in the cam region, so that the necessary freedom of movement of the valve actuator in its sliding or pivot bearing remains preserved. The surround element is therefore arranged, in particular, pivotably about an axis on the valve actuator or on a holder for the valve actuator.
Preferably, the valve actuator has a holder, to which the surround element is connected, in particular movably, and on which a part guided in the sliding or pivot bearing of the valve actuator is held preferably adjustably.
In a first version there is provision for the surround element to have an open loop, the two ends of which are fixed to a holder for the actuator. When the two ends of the open loop pass through one another or, touching one another, project from the cam element, then, because of the flexibility of the material of the surround element, a physical axis may be unnecessary, since the two ends can be jointly bent on both sides to the required extent. For connection to the valve actuator, there is preferably provision for the two ends of the open loop to have insertion orifices for a connecting element. The insertion orifices may be formed by folding round and, depending on the material of the loop, stitching, adhesive bonding, welding or the like of the folded-round end. In a particularly advantageous version, the open loop consists of a band closed on itself which is led to and fro around the cam element and the reversals of which form the insertion orifices.
In a further version, the flexible surround element is designed as a closed loop and is provided with a holder for the valve actuator. The tying of the holder into the loop can in this case be achieved in a simple way by means of a protuberance wherein the holder is arranged. The protuberance is achieved, in particular, by constricting a region of the surround element which forms an excess length, said constriction being, for example, pinched off, stitched or the like.
A closed flexible loop may be formed, for example, by a deformable ring. If the ring consists of plastic, it may also be fiber-reinforced or reinforced on the inside and/or outside with a steel or fabric band. Particularly in this version, an elastomeric plastic could also be used, which is vulcanized onto the band. For the movable connection, there is preferably provision for the ring to have a recess, wherein a bearing pin of the valve actuator is provided, the axis of said pin being parallel to the axis of rotation of the cam element.
Both and open and closed loops can be achieved when the surround element is a rope which, in particular, is inserted into a circumferential groove of the cam element, a roller chain, wherein a joint pin forms the holder of the valve actuator, a strip or band consisting of a sheetlike material, wherein threads or fibers are provided in a textile weave, or a band spring which is manufactured, for example, from a titanium alloy and surrounds the circumferential surface of the cam region in one or more turns one above the other. A closed rope can be slipped, for example, through a lug of the valve actuator or of its holder. In particular, a material formed seamlessly from threads or fibers according to a textile circular technique (circular weaving, circular knitting, etc.) is suitable for manufacturing a closed loop. A circular-woven closed loop preferably contains, as weft threads, aramid fibers which have high length constancy and temperature resistance.
The ends of an open loop element, in particular of a rope, fabric band or band spring, may be clamped on the valve actuator or its holder, be provided with insertion orifices for receiving a connecting pin or cotter or else be designed for fastening in another way.
If a plurality of turns of a band spring are arranged on the cam element, that is to say either a plurality of closed loops in successive sizes or a spiral winding, then, for fixing the holder, the turns may have bores which lie one above the other and the diameter of which decreases from the inside outward. These form a frustoconical orifice, into which can be inserted a frustoconical holder of the valve actuator, said holder requiring no further fixing. The holder may be two-part and, for example, surround a bearing part of the valve actuator, said bearing part being provided with a spherical head behind which engagement is possible. A spherical head at the end of a displaceably mounted valve shank also makes it possible for a valve tappet to be rotatable about its longitudinal axis.
In a further possibility for connecting the surround element of the holder, the holder of the valve actuator has a bearing pin, which is arranged between two turns of the band or of the band spring.
If the surround element consists of a material with a low-friction, if appropriate low-frictionally coated surface, then, if appropriate, lubrication of the sliding surfaces, that is to say of the circumferential surface of the cam element and of the coming-to-bear inner surface of the surround element is not necessary. If lubrication is desired or required, there is preferably provision for the cam element to have, radially to the axis of rotation, at least one oil bore which issues on the circumference of the cam element within the flexible surround element. Since the surround element does not rotate, an external oil supply through the surround element via a flexible line may also be envisaged.
An oil supply which, under some circumstances, is not necessary at all for maintaining the sliding properties may also be used in order to solve a serious problem in previous positive controls when the valve is pressed down faultily during the closing phase, indeed a slightly open valve also not being pressed reliably into the valve seat because of excess pressure in the cylinder. To be precise, an oil film may be built up in the slight gap between the cam element and the surround element, the oil or the oil pressure centering the surround element in relation to the cam element.
Under the action of force from outside (mass inertia forces, etc.), the surround element is pressed out of this force equilibrium, with the result that the oil gap becomes thinner at a specific point. At this point, more oil pressure builds up, which presses the surround element back into the approximately central position again, so that the system is stabilized.
This effect not only is helpful in the rotation of the cam element in the surround element, but may also be used in order to generate a valve closing force, thereby also dispensing, on the valve actuator, with a hydraulic element which, in conventional valves, brings about play compensation in the base-circle region. The oil film also has a damping function, so that the mass forces caused by the acceleration and deceleration of the valve also do not act directly on the cam element, and the engine noises are reduced.
Instead of a sliding oil film, an air cushion surrounding the cam element may also be built up by means of compressed air.
This may be advantageous particularly in the case of a surround element consisting of plastic or of a plastic fabric.
In the valve mechanism according to the invention, owing to the omission of the valve spring and spring plate and to the substantially lighter form of construction of the valve tappet or rocker arm, the masses to be accelerated are reduced. The use of light metals, ceramics or plastic for the valve and/or the valve actuator allows a reduction in the masses to be accelerated and decelerated of 50% to 80% of the value of a valve tappet with a return spring and with hydraulic play compensation. The high values occur particularly in the part-load range, since the valve springs have to be designed for full-load safety. Furthermore, the valve can be made shorter, since the space-consuming valve spring is dispensed with. Moreover, the oil supply gives rise not only to linear contact, but also to surface contact.
The cam element can also be made smaller. The production of plastic cam elements or camshafts produced completely from plastic, for example by injection molding, can also likewise be implemented. The use of other lightweight construction materials for the production of the camshafts or cam elements, for example aluminum, also becomes possible. On account of the reduction in mass and of sliding lubrication, fuel savings of 5% and more are to be expected.
Particularly when valve actuators are actuated jointly, a weak spring may be provided for acting upon each closed valve.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a valve mechanism, in particular for internal combustion engines of motor vehicles, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.